Foams made from alkenyl aromatic polymers (e.g, polystyrene) or polyolefin polymers (e.g. polyethylene and polypropylene) have found extensive use, particularly as packaging, protective, and insulating materials. Polyolefin foams are particularly advantageous due to their tolerance to high service temperatures. Polyolefin foams are commonly manufactured as expanded beads, extruded sheets or extruded boards. The difference between the expanded and extruded foams is that the extruded foams, in the form of continuous sheets or boards, are made in a single-step process; whereas, expanded foams, in the form of discrete, small-size pieces, are made in a multi-step process. Thus, the dimensions of expanded foam are much smaller than those of extruded foam. Furthermore, the expanded foams do not necessarily have to be in the form of beads or peanuts, but can also be made from pellets, rods, platelets, thin sheet or film. For the sake of convenience, the term “bead” or “pellets” will be used throughout this application to imply other shapes in which small, discrete particles of the polymer resin can be used to make expanded foams.
Generally, polyolefin foams in the form of beads or sheets having a thickness of less than about one-half inch can be used to make packaging materials such as containers (cups, bowls, clamshells, picnic chests) for hot or cold beverages or food whereby the beads are fused or the sheet is thermoformed in a mold to form the packaging material of a desired shape. Such foams are also used as protective and cushioning materials for transportation of delicate or shock sensitive articles whereby the foam beads can be used as loose fill dunnage material and thin sheets can be used to provide protective wrapping.
Packaging and insulation foam products with a thickness greater than about 0.5 inch are called planks or boards. Such foam boards are produced in the desired shape and size by direct extrusion and cutting if needed, or by fusing the expanded foam beads. The foam boards can be used for protective packaging by die-cutting the boards to various shapes, for insulation, for dissipating mechanical energy as in automotive parts, or for cushioning floats. It is desirable that the polyolefin foams used in such diverse applications be dimensionally stable; this characteristic is even more desirable for planks or boards.
These and other polymer foams are commonly made using a continuous process where a blowing agent laden molten resin is extruded under pressure through an appropriate die into a lower pressure atmosphere. Alternatively, a batch or staged process can be used, where small polymer beads (also called particles or pellets) are impregnated with blowing agent and then expanded by heating rapidly to a temperature near or above the glass-transition or crystal-melt temperature of the polymer-blowing agent system, or subjected to an external compressive stress at a temperature up to the glass-transition or crystal-melt temperature of the polymer-blowing agent system. Presently, physical blowing agents more commonly used for making polyolefin foams are hydrocarbons, chlorinated hydrocarbons, hydrochlorofluorocarbons, hydrofluorocarbons, or combinations thereof. Hydrocarbons with three or more carbon atoms are considered volatile organic compounds (VOCs) that can lead to formation of smog. Furthermore, some halogenated hydrocarbons are either VOCs or may have high ozone depletion potential (ODP) or global warming potential (GWP), or may be hazardous air pollutants (HAPs) and, at times, may fall into more than one of these categories. Therefore, the use of hydrocarbon and halogenated hydrocarbon blowing agents for preparing polymeric foams is not preferred environmentally and imposes many limitations on the manufacturing process, thus complicating and significantly increasing the cost of manufacturing. For example, polyolefin foams (beads or sheets) are generally made using VOCs such as butanes or compounds such as halogenated hydrocarbons that are HAPs and/or may have high ODP and/or may have high GWP. It is therefore desirable to minimize or eliminate altogether the use of such compounds as blowing agents for preparing polyolefin foams.
Methyl formate is classified as a non-VOC (Federal Register, Volume 69, Number 228, Nov. 29, 2004), is non-HAP, has zero ODP, and negligible GWP. U.S. Pat. No. 6,753,357 to Kalinowski et al., which is incorporated in its entirety herein by reference thereto, describes the use of methyl formate to produce stable, rigid isocyanate/polyol based polyurethane foams. It is noted, however, that such polyurethane foams are thermoset, so as to be made via a cross-linking and curing process. The dimensional stability or instability imparted to the final polyurethane foam product by the nature of the blowing agent therefore is quite different than in the case of polyolefin foams.
U.S. Pat. No. 3,407,151 to Overcashier et al., which is incorporated in its entirety herein by reference thereto, is directed to the production of expanded or expandable thermoplastic materials, particularly crystalline polypropylene, using an insoluble volatile liquid. The process involves the production of a substantially uniform dispersion of fine, discrete masses of blowing agent in a matrix of melted polymer, and extrusion and cooling of the dispersion without substantial coagulation of the fine masses. Methyl formate is included in a list of possible volatile liquids that can be used in this process. Further, Overcashier discloses the use of a single, insoluble liquid and does not disclose or suggest the use of liquid blends.
U.S. Pat. No. 3,976,605 to Matsunaga et al., which is incorporated in its entirety herein by reference thereto, is directed to a foamed plastic of a resin composition including a pullulan type resin and thermoplastic resin. The pullulan resin is a high-molecular weight linear polymeric glucose, having a molecular weight of 10,000 to 5,000,000. The pullulan resin includes both an etherified pullalan and an esterified pullalan. Methyl formate is included in a list of possible volatile liquid foaming agents suitable for use in this process, particularly as an example of a suitable low boiling ester.
U.S. Pat. No. 3,281,259 to Lux et al., which is incorporated in its entirety herein by reference thereto, is directed to a process for rendering the surface of a polyethylene foam sheet printable or receptive to coating by another polymer by creating open cells in the surface of the solid or foamed polyolefin. The process can include i) heating the surface of the foam sheet to produce a surface permeated by a large number of very fine pores; ii) chilling the surface of a foamed polyolefin to produce an unfoamed polyethylene “skin”, which is then rapidly heated to a high enough temperature to create open cells in the surface layer only; or iii) passing a conventional closed cell foamed polyolefin sheet under a microprecision surface grinder so that only the surface layer of polymer is ground away leaving exposed open fragments of bubbles on the surface of the skin. Methyl formate is included in a list of possible volatile liquid foaming agents that can be used in this process.
U.S. Pat. No. 5,026,736 to Pontiff et al., which is incorporated in its entirety herein by reference thereto, is directed to moldable shrunken foam beads. The beads are produced by extruding a mixture of a thermoplastic foamable polymer and a blowing agent, cutting the melt to form foam beads, allowing the beads to shrink to a predetermined density, and, optionally, treating the beads to crosslink the thermoplastic polymer foam by chemical means or electromagnetic radiation. Through proper choice of blowing agents and gas permeation modifiers, the foam beads typically shrink within about 15 minutes after extrusion and cutting. The “shrunken” beads are thus defined as being reduced from their maximum expansion to produce surface wrinkles, dimples or the like. The shrinkage occurs because of the blowing agent's rapid diffusion from the foam structure relative to its replacement with air and/or the low room temperature vapor pressure of the blowing agent, which causes a decrease in the volume of the cells as cooling progresses. Methyl formate is included in a list of possible volatile organic blowing agents that can be used in this process.
U.S. Pat. No. 5,059,376 to Pontiff et al., which is incorporated in its entirety herein by reference thereto, is directed to a process for accelerated removal of residual blowing agents from objects produced from foamed thermoplastic polymers. The process involves subjecting the extruded foam to an elevated temperature to remove a substantial portion of the residual blowing agent and to cause sufficient air to diffuse into the foam so as not to cause substantial collapse of the foam. The foam sheets used in the process described in Pontiff are generally 0.25 inch thick or less. Methyl formate is included in a list of possible volatile organic blowing agents that can be used to produce the polyolefin foams used in the process. The amount of residual blowing agent remaining in the foam after the process will vary with the various blowing agents used, but generally will be less than two weight percent of the foam product.
U.S. Pat. No. 5,391,335 to Sakamoto et al., which is incorporated in its entirety herein by reference thereto, is directed to a process for producing an insulated electric wire having an insulated layer made of a polyolefin foam. The foaming agent is a mixture of at least two compounds, one compound including the group consisting of ethers, esters, ketones and saturated hydrocarbons having boiling points of 100° C. or below and at least one additional liquid compound selected from the group consisting of ethers, esters, ketones, saturated hydrocarbons and alcohols having boiling points higher than 100° C. but not exceeding 150° C. The molten mixture, consisting of resin, blowing agents and additives, is extruded from the die into a cross-head portion which has a conductor-wire continuously guided therein. This allows the molten mixture to coat the periphery of the conductor and foam into a foam-insulating layer.
U.S. Pat. No. 5,565,497 to Godbey et al., which is incorporated in its entirety herein by reference thereto, is directed to a process for the manufacture of a closed cell, rigid, polymer foam which contains a well-dispersed filler. A non-ionic fluorochemical surfactant is used in the formulation to help disperse the filler in order to improve the insulation value of the foam. The filler material may include a variety of inert, non-reactive substances, including particulate clays, carbons, metals, minerals, polymeric materials and diverse chemical compounds and mixtures thereof. Methyl formate is included in a list of possible volatile organic blowing agents that can be used to produce the polyolefin foams used in the this process.
Therefore, a need exists for blowing agents employing methyl formate and environmentally friendly co-blowing agents, preferably non-VOC co-blowing agents, as components of the blowing agent blend to produce stable polyolefin foams, without compromising the product quality in terms of appearance, mechanical or compressive strength, and that enable a cost-effective and versatile manufacturing process.